Two U.S. officials say preliminary evidence from an interrogation suggests the suspects in the Boston Marathon attack were motivated by religion but were apparently not tied to any Islamic terrorist groups, The Associated Press reported Monday.

The two brothers, born in the Chechen region of Russia, practiced Islam.

The U.S. officials spoke Monday on condition of anonymity because they were not authorized to publicly discuss the investigation.

Dzhokhar Tsarnaev could face the death penalty in the April 15 bombings at the Boston Marathon that left three people dead and more than 170 injured, prosecutors announced Monday.

Tsarnaev, 19, is charged with conspiring to use a weapon of mass destruction "against persons and property in the U.S. resulting in death" and one count of malicious destruction of property by means of an explosive device resulting in death, according to the U.S. attorney's office in Massachusetts.

While Tsarnaev and his older brother were on the run, one of them, while brandishing a gun, boasted to a carjacking victim of having committed the bombings, court documents show. A source identified Dzhokhar Tsarnaev as the one who made the boast.

"Did you hear about the Boston explosion? I did that," one of the suspects said, according to the affidavit supporting the charges.

A report detailing the carjacking of the Mercedes SUV, filed with the Cambridge Police Department, says the carjacking victim told police that two men told him "that they were the Boston Marathon bombers and would not kill him because he wasn't American."

He managed to escape, the report says. The name of the victim is not given.

A clerk in the Office of the Circuit Executive confirmed the filing Monday of charging documents against Tsarnaev. He was charged at his bed in Beth Israel Deaconess Hospital in the presence of a magistrate judge.

Tsarnaev, described as alert and able to respond, was advised of his rights and the charges against him, and prosecutors stated the maximum penalties and moved for detention. Tsarnaev agreed to voluntary detention without prejudice, meaning he can still seek release at a future court hearing. He declined to answer bail questions, and agreed to a probable cause hearing on May 30.

The statutory charges authorize a penalty, upon conviction, of death or imprisonment for life, prosecutors said.

"Although our investigation is ongoing, today's charges bring a successful end to a tragic week for the city of Boston and for our country," U.S. Attorney General Eric Holder said in a news release from the office of Carmen Ortiz, U.S. attorney for the District of Massachusetts. "Our thoughts and prayers remain with each of the bombing victims and brave law enforcement professionals who lost their lives or suffered serious injuries as a result of this week's senseless violence."

A White House official said the surviving suspect in the attack will not be tried as an enemy combatant in a military tribunal, according to The Associated Press.

White House spokesman Jay Carney said Tsarnaev will be prosecuted in the federal court system, and President Barack Obama's entire national security team supported the decision, the AP reported.

Tsarnaev was in serious but stable condition, according to the FBI. He had been unable to speak due to a neck and throat injury, the source said. He was also intubated and breathing with the help of a respirator and earlier, officials had said he was not in any condition to be questioned.

It was not clear, however, whether what Tsarnaev was writing was of any value.

A source involved in the investigation said late Sunday the motorist carjacked by Tsarnaev and his older brother, Tamerlan Tsarnaev, 26, stated the young men planned to drive to New York. The source called such a trip "extremely unlikely" given the extent of the manhunt for the suspects after they fatally shot an MIT police officer, carjacked the motorist, and led police on a chase that ended in a shootout in Watertown, Mass. That assessment was proved correct by the events that followed Friday. However, that tip prompted authorities in Connecticut to be on the lookout for the brothers.

The possibility that the suspects might have been headed to New York prompted local authorities to issue an all-points bulletin in the metro area even as officers in Watertown, Mass., conducted a house-by-house search, according to a source.

Authorities believe Dzhokhar Tsarnaev may have tried to shoot himself before he was taken into custody Friday night because of the trajectory and location of the bullet wound in his neck, a source familiar with the investigation said Sunday.

The shot was fired at close range, the source said, suggesting the wound was self-inflicted. He was found hidden in a boat in a backyard in Watertown, bloody and injured. He was able to step out of the boat before being taken into custody after a brief exchange of gunfire with police and then negotiation with FBI agents, said Boston Police Commissioner Edward Davis.

As he lay on a stretcher in the back of an ambulance on the way to hospital, the suspect was mouthing curse words, either to himself or to the armed authorities standing watch in the ambulance as paramedics treated him, the source said.

Boston Mayor Thomas Menino said Sunday authorities are now convinced Dzhokhar Tsarnaev and his brother, who was killed in a shootout with police Friday morning, acted on their own in the bombing.

Police Commissioner Davis said investigators also have found a circuit board with a wire slotted through it at the bombing scene on Boylston Street and the area had been swept clean of explosives. Menino said that area is still under the control of the FBI, but will be released to the city soon and a five-phase restoration is planned.

Hundreds of FBI agents and other investigators have interviewed more than 100 people who knew the brothers, the source said. They are looking for any possible links to foreign extremists and seeking to understand how the plot developed and if anyone helped them, the source said.

He confirmed that Tamerlan Tsarnaev had video of a known jihadist on his YouTube channel, footage the source had viewed and characterized as "extremely inflammatory."

Menino had said earlier it has become clear that the older brother, Tamerlan, was the more influential, describing him as the "leader" and Dzhokhar as a "follower."

The older brother "read those magazines that are published on how to create bombs," Menino said.

Investigators are also tracking the brothers' online activities, phone calls and text messages, examining hundreds of pieces of evidence -- including bomb components, unexploded devices as well as the brothers' vehicles, guns, and financial records seized from various sites across Boston and surrounding suburbs, authorities said.

The blasts set off near the finish line of the marathon killed three people and injured more than 180.

Sunday morning, Davis said on the CBS News show, "Face the Nation" that the crime scene from the early Friday morning firefight in Watertown that killed Tamerlan Tsarnaev was "littered" with unexploded improvised explosive devices.

Davis said it's his belief that the brothers were "going to attack other individuals" -- and Davis said that's based on the evidence at the scene and the firepower that the brothers had.

Two men with connections to the brothers were taken into custody Saturday in New Bedford, Mass., and eventually arrested and detained on immigration violations by Immigration and Customs Enforcement, that agency announced.

The source said those arrests were made in an abundance of caution because the men were friends and former roommates of Dzhokhar Tsarnaev and had regular contact with him. Officials want to question them at greater length about whether they knew anything about the brothers' plans or provided any help to them.

Sunday morning, Boston Cardinal Sean O'Malley implored hundreds of parishioners at a Mass at the Cathedral of the Holy Cross to forgive the suspected Boston Marathon bombers and seize on the goodwill born of the tragedy.

"We must be people of reconciliation -- not revenge," he told about 800 parishioners during a service that honored four killed in last week's marathon bombings and their aftermath. Candles lighted an altar with photos of the dead victims, including the three killed Monday in the bombing, and a police officer killed Friday in a shootout.

Speaking to reporters after the service, O'Malley reiterated the church's opposition to the death penalty. "Forgiveness does not mean that we do not realize the heinousness of the crime. But in our hearts, when we are unable to forgive, we make ourselves a victim of our hatred. Forgiveness is part of our obligation of disciples of the Lord."

"It does seem to be pretty clear that this suspect took the backpack off, put it down, did not react when the first explosion went off and then moved away from the backpack in time for the second explosion," Patrick said. "It's pretty clear about his involvement and pretty chilling, frankly."

Patrick said it's hard to imagine why someone would deliberately harm "innocent men, women and children in the way that these two fellows did."

The FBI had disclosed this past week that it had questioned Tamerlan Tsarnaev in 2011 regarding possible terrorist connections, but found nothing and took no action against him.

Sen. Charles Schumer (D-N.Y.) said Sunday he understood that the FBI didn't find any troubling evidence during its initial contact with the suspect in 2011 -- prompted by concerns passed along by Russian authorities -- but he noted that was before Tamerlan Tsarnaev made a suspicious overseas trip.

"Then he went to Russia and he came back and almost immediately put very radical things on his website," Schumer said of the elder Boston bombing suspect. "Why didn't the FBI, having known that he was someone a foreign government at least thought might be dangerous, go interview him again?"

Schumer, speaking during an unrelated news conference, said that the federal death penalty law he helped co-author in the 1990s would be "appropriate" in the case of the surviving suspect.

On Saturday, Rep. Peter King (R-Seaford) had also questioned the FBI's actions.

Meanwhile, rebels in Russia's North Caucasus region denied any link to the April 15 attack on the Boston Marathon in a statement posted on the vdagestan.com website.

"The Caucasian Mujahedeen are not fighting with the United States of America," according to the statement, which was attributed to unidentified Mujahedeen commanders in Dagestan. "We are at war with Russia, which is responsible not only for the occupation of the Caucasus, but also for heinous crimes against Muslims."

An FBI-led probe is looking into a six-month trip suspect Tamerlan Tsarnaev, an immigrant of Chechen descent, took around two years ago to Chechnya and neighboring Dagestan, both regions of Russia that have been embroiled in Islamist separatist movements.

In Medford, Mass., bombing victim Krystle Campbell was eulogized Monday after a funeral procession from the funeral home where she was waked to St. Joseph Roman Catholic Church. Services concluded with burial at Oak Grove Cemetery.

The Associated Press reported that doctors said the Boston transit police officer wounded in a shootout with the bombing suspects had lost nearly all his blood and his heart had stopped from a single gunshot wound that severed three major blood vessels in his right thigh.

Surgeons at Mount Auburn Hospital in Cambridge say Richard Donohue, 33, is in stable but critical condition. He is sedated and on a breathing machine but opened his eyes, moved his hands and feet, and squeezed his wife's hand Sunday.

Emergency workers started CPR on the scene to restart his heart. Doctors say he is expected to make a full recovery and that nerves and muscles in his leg are intact.

Transit officials say Donohue had gotten out of his cruiser and was shooting at the suspects when he was hit late Thursday night in a gun battle in Cambridge.

"He went in there and engaged people who were shooting at his fellow officers," Donohue's brother, Edward, said at a news conference Sunday at the hospital. "I cannot describe the pride I have," said the younger Donohue, who is a patrolman for the Winchester Police Department.

The fellow officers included a friend, MIT police officer Sean Collier, who authorities say was shot to death at the beginning of the brothers' rampage earlier in the night.

Richard Donohue has been a transit officer for three years, said Paul MacMillan, chief of the Massachusetts Bay Transportation Authority.

The University of Massachusetts-Dartmouth, where Dzhokhar Tsarnaev was a student, reopened Sunday at noon, university officials announced on the school's website.